A coil current flows in a coil arrangement, which has an inductance and which is part of a magnet system containing coil cores and/or pole shoes. The coil current is clocked such that the coil current is positive in a first half of a period and has a constant first electrical current end value and negative in the second half of the period with a constant second electrical current end value, which is of equal magnitude to that of the first electrical current end value.
The coil cores and/or pole shoes of the magnet system are, most often, formed of a soft magnetic material. There are, however, also magnet systems with ferromagnetic coil cores.
In the case of both types of magnet systems, there are induced in the magnet system, due to the rise and fall of the coil current, eddy currents, which prevent the rise of the magnetic field from following exactly the rise of the coil current, such as would be the case, if the coil cores and/or pole shoes were absent. Rather, the rise of the magnetic field is delayed and flattened, compared with that of the coil current. In such a situation, an exact control of the coil current to a constant end magnitude is required.
European Patent EP 0 969 268 A1 discloses a method for controlling coil current, wherein, first of all, an overvoltage is provided for a predetermined time interval and after this time interval a feedback control occurs. This method has, as a rule, proved itself in practice. If, however, due to fluctuations or disturbances, there is a shifting of the starting point, then, for example, a higher maximum electrical current level would be produced in the constant time interval and this could, in given cases, lead to overloading.
Moreover, also known are methods, which have a diagnostic function to check for a maximum electrical current level. In these methods, however, the electrical current level serves not as a control variable, but, instead, is only taken into consideration as a value for checking the control unit.